Based on the new class of superconductors, henceforth referred to as high-Tc superconductors, which were discovered by Bednorz and Müller and disclosed in their article “Possible High-Tc Superconductivity in the Ba—La—Cu—O System”, Zeitschrift für Physik B, Condensed Matter, Vol. B64, 1986, pp.189-193, a variety of superconducting wires, cables and tapes have been developed for the transport of electrical current. A key parameter defining the performance and thus the economic benefit of these conductors is given by their so-called critical current density, which is the maximum current these conductors can carry as so-called supercurrents in the superconducting state divided by the cross-sectional area A of the superconductor. The critical current density is a specific property for a given superconductor, and, for the practical use of a superconductor, one aims to maximize the critical current density.
Chaudhari et al. have taught in their article “Direct Measurement of the Super-conducting Properties of Single Grain Boundaries in YBa2Cu3O7−δ”, Physical Review Letters, Vol. 60, 1988, pp.1653-1655, that the limiting factor for the critical current density of polycrystalline high-Tc superconductors is the electronic behavior of the boundaries formed by the crystalline grains of these materials. It was shown that the critical current densities of these grain boundaries are smaller by one to two orders of magnitude than the critical current densities of the grains abutting the grain boundaries.
Further, Dimos et al. have taught in their publication “Superconducting Transport Properties of Grain Boundaries in YBa2Cu3O7−δ Bicrystals”, Physical Review B, Vol. 41, 1990, pp. 4038-4049, that superconducting grains with a small misorientation (below typically 8° to 10°) behave as strongly coupled superconductors whereas larger misorientations (also called large-angle grain boundaries) are weakly coupled, showing Josephson junction-like properties. The teaching by Dimos et al. is the finding that the detrimental effect of the grain boundaries can be reduced by aligning the superconducting grains with respect to their crystalline main axes.
Following this proposal, wires and tapes of high-Tc superconductors have been fabricated, the critical currents of which are enhanced by aligning the superconducting grains by a variety of means, such as rolling processes or ion beam assisted techniques. Although these technologies have lead to the fabrication of high-Tc superconductors with current densities of the order of 100 000 A/cm2 at temperatures of 4.2 K, it remains desirable to fabricate high-Tc superconductors with still higher critical current densities or with processes which are less costly and faster than the known ones.
Mannhart and Tsuei have revealed in their publication “Limits of the Critical Current Density of Polycrystalline High-Temperature Superconductors Based on the Current Transport Properties of Single Grain Boundaries”, Zeitschrift für Physik B, Vol. 77, 1989, pp 53-59, that the critical current density of a three-dimensional conductor can exceed by an order of magnitude the given critical current density of the grain boundaries. This approach, illustrated in FIG. 1, is based on the fact that the critical current of conductor is a function of the grain boundary critical current density as well as of the effective area A′ of the grain boundaries, which may be much larger than the cross-sectional area A of the conductor. The effective grain boundary area A′ may be enhanced, e.g., by adjusting the microstructure of the superconductor such that the grains have a large aspect ratio, the long sides of the grains being oriented predominantly parallel to the supercurrent flow. In their publication, Mannhart and Tsuei also revealed a procedure to calculate the critical current as a function of the grain aspect ratio. These calculations show that the critical current strongly increases with the aspect ratio of the grains, being ultimately limited only by the intragrain critical current densities.
It was pointed out by Mannhart and Tsuei that large critical current densities may be attainable by using superconducting films with aligned needle-shaped grains. Although this proposal shows the right way to fabricate tapes with large critical current, despite more than 10 years of intense R&D efforts on this problem, no way was found to fabricate such conductors.
However, in accordance with the proposal of Mannhart and Tsuei, cables based on high-Tc superconductors such as Bi2Sr2Ca1Cu2O8+δ or Bi2Sr2Ca2Cu3O10+δ (BSCCO) so-called first generation cables have been fabricated, which use superconductors which contain platelet-like grains arranged in such a manner that huge effective grain boundary areas are obtained (see FIG. 3 for an illustration). This is described by Mannhart in “Critical Currents in High-Tc Superconductors” in “Physics of High Temperature Superconductors”, Proceedings of the Toshiba International School of Superconductivity, Kyoto, Japan, Jul. 15-20, 1991, Springer Series in Solid State Sciences, Vol. 106, 1992, 367-393 (1991). This method, also called powder in tube method, uses large grain boundary areas in bulk BSCCO with success. In this method, Ag-tubes are mechanically filled with BSCCO-powder. The tubes are then drawn and rolled into a final, tube- or tape-like shape and then fired for reaction and annealing. The large grain boundary areas in the bulk BSCCO-filling of the tubes originate from the platelet-like microstructure of the very anisotropic BSCCO-compounds. Unfortunately the material cost involved in the powder in tube process are so high that this technology cannot be commercially competitive.
Another method to fabricate conductors made of high-Tc superconductors avoids the use of bulk materials and silver (as done in the powder in tube process), but instead enhances the grain boundary critical current densities by aligning the grains by epitaxially depositing superconducting films. It is this technology, called coated conductor technology, about which the invention is concerned. Tapes fabricated by the coated conductor technology are also called conductors of the second generation, as this process has the potential to solve the cost-problem of the first-generation conductors. The required grain alignment (texturing) is achieved, for example, by depositing the superconductor on a template that has a textured surface (an overview of coated conductors is provided by D. Larbalestier et al., “High-Tc Superconducting Materials for Electric Power Applications”, Nature, Vol. 414, 2001, pp 368-377 and references therein). Deposition is usually done using standard vapor phase deposition techniques such as sputtering, laser deposition, or thermal evaporation. Recently, non-vacuum techniques like sol-gel methods or dip coating have also been used for this purpose.
At present, coated conductors are fabricated by using predominantly three different processes. In all of them the conductor typically consists of a substrate, for example a metallic tape, a buffer layer system which usually is based on a series of oxide films, a layer of a high-Tc superconductor such as YBa2Cu3O7−δ, and possibly several doping and capping layers. In contrast to the polycrystalline superconductor of the powder-in tube conductors, the grains in the superconductor layer of the coated conductors usually form a two-dimensional network, because the high-Tc superconductor is in most cases epitaxially deposited as a polycrystalline film.
The first technique to produce coated conductors to be described here is known as the rolling assisted biaxially textured technique (RABiTS). When tapes of nickel based alloys or similar materials are rolled and suitably heat treated, the Ni-grains become textured along two of their main crystal-axes, so that grain boundaries are aligned in all directions. This makes the metallic tape a useful substrate for the fabrication of a coated conductor. On the surface of the tape a buffer layer, usually composed of CeO2 and Y-stabilized ZrO2 is grown. On top of this buffer layer a high-Tc material, typically YBa2Cu3O7−δ, is deposited as film. These epitaxial films reproduce the microstructure of the buffer layer, which in turn has replicated the microstructure of the nickel alloy substrate. The thickness of the superconducting films is in the range of a few microns, the entire tapes being 25-50 micrometers thick. This process, known as the rolling assisted biaxially textured technique (RABiTS) is capable to produce low-angle boundaries (2°-5°), consequently, the critical current density is relatively high, reaching values above 105 A/cm2 at 77 K in one Tesla.
Texturing can also be induced by ion beam assisted deposition (IBAD) or deposition under a glancing angle, which is the so called inclined substrate deposition method (ISD). In these techniques the buffer layer is textured during growth. This is done in the ISD-process by using a shallow angle between the incoming beam of adatoms and the substrate surface, and in the IBAD technique by irradiating the growing film with additional ions. The critical current densities of the superconducting films, having again a typical thickness of a few micrometers, exceed 106 A/cm2 at 77 K and zero external magnetic field. A limiting factor for applications of these processes is their low speed, caused by the cumbersome alignment processes.
The coated conductor processes have not been able to fabricate grains with enhanced aspect ratios (see, e.g. FIG. 1d of D. Larbalestier et al., “High-Tc Superconducting Materials for Electric Power Applications”, Nature, Vol. 414, 2001, pp 368-377),
Immense efforts are devoted in Asia, the US and in Europe to improve the coated conductor processes. Despite these efforts, possible market applications are at best several years away. The reason is that the texturing of the tapes is a tedious and costly process. Due to this, the maximum length of the coated conductors produced today is in the range of several meters only, and no practical way has been found to produce larger length at competitive costs. It is clear that the commercial breakthrough of conductor conductors could be obtained if the current density of the cable could be enhanced significantly for a given grain alignment. Therefore such methods are sought with great intensity as described by P. Grant in “Currents without Borders” Nature Vol. 407, 2000, pp 139-141. If such a method was found, one could benefit for given production costs from an enhanced critical current, or, if the grain alignment was relaxed, from standard critical currents at much lower costs.
The present invention provides the solution to his problem.